This invention relates to temperature control, both in terms of systems and methods, intended for lowering the temperature of an enclosure or partial enclosure and/or any objects within it from a higher actual value to a desired lower value and maintaining it substantially constant at the desire lower value for any required periods by flowing through the enclosure a liquefied gas in either the liquid or the vaporized state as the application demands. The cooling effect of the liquefied gas, which is drawn from a suitable storage container, is determined by controlling the rate at which it flows through the enclosure and the rate at which it is dissipated, e.g. to atmosphere, via a venting orifice, the two rates being so coordinated in response to the temperature sensed within the enclosure as to attain and -maintain the desired lower temperature.
In the present specification, the term "lower" in the phrase "the desired lower temperature", or the equivalent phrase "the desired lower value" following a prior mention of the term "temperature", shall be construed with reference to an assumed "actual higher temperature", e.g. if an object is at 50.degree. C. and it is required to cool it down to 5.degree. C., the former value is the "actual higher temperature" and the latter the "desired lower temperature". The use of a liquefied gas coolant does not necessarily mean that the desired lower temperature is a sub-zero temperature.
A commercially available liquefied gas is usually referred to by the name of the gas preceded by the term "liquid", e.g. liquid helium. This convention will be followed in the present context and, by extension, the phrase "liquid gas" will be used as a generic expression to refer to liquefied gas either in the liquid or vaporized state, unless the context specifies one of the two states.
Liquid nitrogen, which is readily available at a comparatively low cost, is particularly suitable as a cooling medium (hereinafter coolant) in many applications where the temperature of the enclosure and/or any objects within it must be depressed down to -100.degree. C. or lower from a higher temperature; but even where the desired lower temperatures are not extreme, or, indeed, are above zero, liquid gas cooling may be indicated if high cool-down rates are required. If both high rates and very low temperatures must be attained, there are almost no practical alternatives, especially if high thermal capacitances are also involved.
In what follows specific reference will be made to liquid nitrogen as a coolant, but that shall not detract from the generality of suitable liquid gases that could be used as alternative coolants.
A problem encountered with liquid gas cooling is how to maintain a desired lower temperature within reasonably close limits, say, 1.degree. C., when virtually the only expedient form of control is achieved by regulating the flow of the coolant, which flow is itself subject to variables, some of which are unpredictable. Before dwelling on the problem, it may be helpful to review briefly the construction of the vessel in which liquid gases are commercially supplied.
If we do take liquid nitrogen as an example, this product is made available commercially in a rather large stainless steel storage container called a Dewar (hereinafter the Dewar) which comprises an inner cylinder suspended from its top end within and spaced from an outer cylinder, the interspace being evacuated for good thermal insulation in order to minimize heat flow from ambient and keep vaporization of the liquid nitrogen within acceptable limits. In a typical arrangement, a manifold into which are screwed taps is provided at the top of the Dewar: one, the supply tap, communicates with a long narrow pipe reaching close to the bottom of the inner cylinder and is intended for supplying liquid nitrogen to a utilization circuit; the other communicates with a short pipe which only reaches just beyond the upper end wall of the inner cylinder and serves to draw vaporized liquid nitrogen instead, if so required by the user, apart from enabling the user to pressurize the Dewar with nitrogen gas to a pressure between 20 and 25 p.s.i. (i.e. between approximately 137 and 172 Kilopascals), unless self-pressurization has been provided for by the manufacturer of the Dewar. The pressure exerted by the vaporized liquid gas within the Dewar which bears on the liquid gas and forces it up the long pipe towards the supply tap shall hereinafter be referred to as Dewar pressure. A blow-off valve venting to atmosphere prevents any excess pressure building up. The user may in addition reduce the Dewar pressure by venting the coolant through the second mentioned tap. When the Dewar is not being used, some vaporized gas will be relieved from time to time, the frequency depending on ambient temperature, setting of the blow-off valve and Dewar insulation.
For the purpose of conveying liquid gas from the supply tap of the Dewar to a connection with the utilization circuit, a thermally insulated pipe of suitable length and bore is used that is usually referred to as a "transfer line". If the coolant in the transfer line has been static or flowing at a low rate for some time, the coolant will be in the vaporized state because under such conditions the cooling will not be adequate to maintain the liquid state against the warming effect on the coolant of ambient heat leaking through the thermal insulation of the transfer line. It follows that after first opening the supply tap of the Dewar the coolant may issue from the transfer line in the vaporized state until an appropriate coolant rate has been maintained long enough to cool down the transfer line to the point where vaporized gas is replaced by liquid gas. Naturally, the time required for the change over will depend on the flow rate. In fact, in all but the most demanding applications it is possible to switch over to vaporized liquid gas cooling by so adjusting the coolant rate that at a given ambient temperature it just fails to cool the transfer line sufficiently to effect the change over.
The transfer line may be supplied by the manufacturer of apparatus in which liquid gas cooling is utilized. Its characteristics will therefore be such as to allow the coolant flow rate to reach the maximum value likely to be required at the point of connection to the utilization circuit, assuming a standard Dewar pressure, such as between 20 and 25 p.s.i.. Of course, non-standard transfer lines and Dewar pressures may be substituted if desired and the coolant flow rate adjusted accordingly.
It will be appreciated from the above background details that cooling an enclosure requires a flow of coolant to be maintained through the enclosure. If we assume that a given maximum flow rate will be required to achieve a reasonably fast initial cool-down to the lowest temperature in the design range of a cooling system, that flow rate may be maintained continuously until the required temperature has been attained and thereafter on an intermittent basis via controlling means responsive to a temperature sensor within the enclosure, said means being adapted to ensure that the cooling effect is just adequate to balance the effect of heat transfer from ambient.
In a typical known arrangement, the coolant is passed through a solenoid valve which is either fully open or fully closed, depending on a command signal responsive to the temperature sensor. When the enclosure is at ambient temperature and the desired lower temperature is many tens of degrees below zero, the command signal causes the solenoid valve to stay open until the desired lower temperature is reached, when the solenoid valve begins to act intermittently.
Clearly, if a rapid cool-down to a very low temperature is required, the maximum flow rate must be comparatively high and yet when the low temperature has been reached the cooling effect required to maintain it is relatively small. To decrease the effect sufficiently, the solenoid valve must be operated at a very high rate of intermittence, which limits its life very considerably, affords poor temperature control, is inefficient in terms of coolant consumption and gives rise to objectionable noise. These are serious drawbacks of the prior art.